Your heart is not a metronome

What heart rate variability can - and can’t - tell you about stress, recovery and longevity

Heart rate variability - usually shortened to HRV -has become the latest number for health-conscious people to monitor.

Smartwatches, rings and fitness bands now produce daily readiness scores based partly on HRV. People compare their numbers, celebrate a rise and become concerned when the score drops overnight.

But is HRV truly an important marker of health and longevity? And should we all be trying to increase it?

The answer is nuanced.

HRV can provide useful information about how your body is responding to sleep, exercise, illness, alcohol and other forms of physiological load. But it is a marker, and not a diagnosis, a report card or a longevity score.

What is heart rate variability?

Your heart is not designed to beat like a metronome.

Even when your average heart rate is 60 beats per minute, the gap between each heartbeat is not exactly one second. One interval may be 970 milliseconds, the next 1,030 milliseconds and the next 990 milliseconds.

Those tiny beat-to-beat differences are your heart rate variability.

HRV reflects the continual adjustments being made by the autonomic nervous system, the network involved in regulating heart rate, blood pressure, breathing, digestion and other processes that occur without our conscious input. 

At rest, HRV is particularly influenced by the vagus nerve and breathing. Heart rate usually accelerates slightly as we breathe in and slows as we breathe out.

HRV is better understood as a measure of physiological adaptability than as a direct measurement of stress or of the vagus nerve itself. Its meaning also depends on the metric used, the length of the recording and the circumstances in which it was measured.

Is a higher HRV always better?

In broad population studies, people with greater resting HRV often have better cardiorespiratory fitness and healthier balance between the sympathetic and parasympathetic nervous system.

HRV varies considerably between individuals and is affected by:

• Age and genetics

• Sex

• Fitness and training history

• Heart rate

• Sleep and circadian rhythms

• Breathing rate

• Posture and measurement time

• Alcohol

• Acute illness

• Medications

• Physical and emotional load

HRV generally changes with age, and the measurement method itself makes a significant difference. A five-minute reading can’t be compared with an overnight average, and one company’s proprietary readiness score cannot be directly compared with another’s.

That’s why comparing your number with your partner, running companion or favourite online longevity expert is rarely useful.

Your trend is generally more informative than somebody else’s score.

Is HRV important for longevity?

There is a genuine association between HRV and long-term health outcomes.

A comprehensive meta-analysis involving 38,008 participants found that lower HRV measurements were associated with higher all-cause and cardiac mortality across both general and clinical populations.

That makes HRV an interesting potential marker of health and ageing.

It doesn’t prove that deliberately increasing a wearable HRV score will make someone live longer.

Lower HRV is actually reflective of the combined effects of cardiovascular disease, lower fitness, metabolic dysfunction, poor sleep, chronic stress, inflammation, smoking, alcohol, illness or ageing. Improving those underlying factors can improve both HRV and health, but HRV is just the messenger.

In other words:

HRV can be a longevity marker. It isn’t yet proven to be a longevity lever.

For longevity, we would still place established priorities such as regular physical activity, cardiorespiratory fitness, muscle strength, blood-pressure control, avoiding smoking, healthy metabolic function and sufficient sleep well ahead of trying to manufacture a particular HRV number.

Why did my HRV fall last night?

A lower-than-usual reading can occur after:

• A poor or shortened night’s sleep

• Alcohol

• A demanding workout

• Inadequate recovery

• Travel or a disrupted routine

• Pain

• Emotional strain

• A late or unusually heavy meal

• Dehydration

• The early stages of an infection

The important phrase is lower than usual.

One low reading doesn’t diagnose overtraining, burnout, illness or poor health. It is purely an appropriate short-term response to what your body has recently experienced.

Rather than asking, “What is wrong with me?”, it may be more useful to ask:

What load might my body be responding to today?

Can a wearable measure HRV accurately?

Wearables have made HRV accessible, but they are not all measuring it in the same way.

Clinical HRV is traditionally calculated from the electrical intervals recorded on an ECG. Most watches and rings instead use optical pulse signals. They can perform reasonably well under quiet, stationary conditions (particularly during sleep) but movement, poor skin contact, irregular beats and device algorithms can affect results.

A 2025 study comparing nocturnal HRV from five popular consumer wearables with ECG found meaningful variation in performance between devices. Research also cautions against treating pulse-derived variability and ECG-derived HRV as completely interchangeable.

For everyday use, the most sensible approach is to:

• Use the same device

• Measure it under similar conditions

• Establish a personal baseline

• Look at trends over several days or weeks

• Interpret HRV alongside sleep, resting heart rate, exercise and how you actually feel

Your wearable can offer a useful clue, but it shouldn’t be given the final word.

How can HRV be improved?

The most useful approach is not to “hack” HRV. It is to improve the physiology that HRV is reflecting.

Exercise regularly

Regular exercise is one of the more credible ways to improve autonomic regulation over time.

A hard workout can temporarily lower HRV. But this doesn’t mean the exercise was harmful. Training is a controlled physiological stress; adaptation occurs when the training stimulus is followed by sufficient recovery.

Protect your sleep

Sleep deprivation can reduce vagally mediated HRV and disturb cardiac autonomic regulation. A 2025 meta-analysis found a significant reduction in HRV following sleep deprivation, although not every HRV measurement changed consistently.

The practical priorities remain wonderfully unexciting:

• Allow enough time for sleep

• Maintain a reasonably consistent routine

• Address persistent insomnia

• Investigate significant snoring, witnessed apnoeas or unexplained daytime fatigue

Consider the effect of alcohol

Alcohol commonly raises overnight heart rate and reduces resting HRV, particularly as the dose increases. Acute alcohol exposure has been associated with reduced HRV in healthy people, while alcohol dependence is also associated with lower HRV.

For people who enjoy tracking data, comparing several alcohol-free nights with their usual pattern can be an interesting personal experiment.

Use slow breathing appropriately

Gentle, slow-paced breathing (around six breaths per minute) can increase HRV during and shortly after the exercise.

A meta-analysis of 31 studies found that slow breathing produced immediate improvements in RMSSD and SDNN, as well as a small reduction in heart rate. The evidence for persistent, long-term cardiovascular effects remains less certain.

Try breathing gently for five minutes:

• Approximately five seconds in

• Approximately five seconds out

• Without straining, gulping air or forcing the breath

One important qualification: slow breathing naturally exaggerates the respiratory variation between heartbeats. A dramatically higher HRV reading during the exercise does not mean that biological ageing has suddenly been reversed.

Recover intelligently

Recovery does not always mean doing nothing.

On a day when HRV is lower and your body feels depleted, it may be reasonable to replace a demanding training session with walking, easier aerobic exercise, mobility work or a lighter strength session.

But HRV shouldn’t be allowed to override symptoms  or common sense. Equally, a low number shouldn’t be over-interpreted to create fear of movement when you otherwise feel well.

When should you speak with your doctor?

An isolated HRV reading rarely requires medical investigation.

Clinical review is appropriate when wearable changes occur alongside:

• Palpitations or a persistently irregular pulse

• Fainting or near-fainting

• Chest pain

• New breathlessness

• A marked reduction in exercise tolerance

• Repeated irregular-rhythm notifications

• A persistent and unexplained change in resting heart rate

HRV can’t diagnose an arrhythmia, heart disease, overtraining or an autonomic disorder.

The bottom line

HRV is one of the more interesting measurements offered by modern wearable technology.

Used sensibly, it may help you notice how your body responds to sleep, exercise, alcohol, illness and recovery.

Used poorly, it can become another daily score to chase and another source of unnecessary health anxiety.

Compare yourself with yourself. Watch the trend. Consider the context. Improve the fundamentals.

Don’t chase the metric. Improve the system and allow the metric to show you how that system is adapting.